Positive-working photoresist compositions generally comprise a composition containing an alkali-soluble resin and a light-sensitive substance based on a naphthoquinonediazide compound. For example, U.S. Pat. Nos. 3,666,473, 4,1115,128, 4,173,470, etc., describe "novolak type phenolic resin/substituted naphthoquinonediazide compound", and L. F. Thompson, Introduction to Microlithography (ACS, No. 219, pp. 112-121) describes an example of a "novolak resin composed of cresol-formaldehyde/1,1-naphthoquinonediazidesulfonate of trihydroxybenzophenone" as the most typical composition.
Novolak resins used as binders are particularly useful in this environment, since they can be dissolved in an alkaline aqueous solution without swelling and, when a resulting image is used as a mask for an etching process, they provide a high resistance particularly against plasma etching. Naphthoquinonediazide compounds used as light-sensitive substances, which themselves function as a dissolution-preventing agent to reduce alkali solubility of the novolak resin, are peculiar in that, when decomposed by irradiation with light, they produce an alkali-soluble substance which increases the alkali solubility of the novolak resin. Naphthoquinonediazide compounds are particularly useful as light-sensitive substances for a positive-working photoresist due to this significant change in properties caused by light irradiation.
Many positive-working photoresists containing a novolak resin and a naphthoquinonediazide light-sensitive substance have been developed and put into practice in view of the above properties, and sufficient results have been obtained in working with a line width of about 1.5 .mu.m to 2 .mu.m.
However, due to the progressively increasing degree of integration of IC circuits, particularly in the manufacture of semiconductor bases for super-LSI, etc., working of superfine patterns having line widths of not more than 1 .mu.m have been required. In such uses, photoresists having high resolving power, high pattern-reproducing accuracy capable of accurately reproducing an exposure mask image, and high speed in view of high productivity have been required. However, the above-described conventional positive-working photoresists fail to meet these requirements.